Inhomogeneous substrate analysis using EM300 backscatter imagery

Backscatter reflectivity from multibeam echo-sounders provides a powerful tool to efficiently characterize seafloor substrates. A comprehensive EM300 bathymetric and backscatter survey has been completed of Cook Strait, in central New Zealand. This paper presents a detailed analysis of the realtime corrections applied to the raw EM300 multibeam data and additional corrections required to compute angular variations of the backscatter strength. The corrections, including the local absorption coefficient, the influence of seafloor topography and sound refraction in the water column, are determined for different Cook Strait seafloor substrates. Modifying MB-System software code, we extracted the backscatter signal parameters in order to quantify the raw backscatter strength and apply additional processing. Profiles of backscatter strength versus incidence angle were computed for a variety of sites characterized by flat seafloor and homogeneous substrates, and for which ground-truth data were available. For each homogeneous site, different but characteristic backscatter profiles are observed that can be interpreted in terms of sediment facies. To analyze heterogeneous substrates, we present a statistical technique, based on a 3-dimensional distribution of (incidence angle, backscatter strength) couples that preserves geological information of the substrate components. This analysis, using backscatter data acquired on a submarine volcano, north of New Zealand, clearly differentiates soft sediments and lava flows within a heterogeneous substrate.     

The motion of a 9.1 m yacht hull buoy moored in open sea

A 9.1 m yacht hull was instrumented to measure its three-dimensional motion when moored in the open sea. The hull was deployed on three occasions for a total period of about 2 months and encountered a wide range of wind-wave conditions including a strong gale. The data have been analysed to give the response for each component of motion in terms of amplitude and relative phases. The hull motion is compared spectrally to the waves observed by a nearby Waverider. The hull was found to behave as a surface-keyed buoy, with a well defined response for a wide range of conditions. The presence of a resonance in pitch and roll is evident in the data with typical rms values being 5° and 10°, respectively, for significant wave height of 5 m. From data on the mooring dynamics it is concluded that the peaks observed in the tension are a result of the viscous drag opposing the change in the catenary of the mooring and the slow drift oscillations of the buoy. This type of hull is a versatile and economical candidate as a platform for meteorological and oceanographic instrumentation.     

Population structure of Haliotis rubra from South Australia inferred from nuclear and mtDNA analyses

1Introduction ThemajorityofAustralia’sabalonefisheryex ports(5.135kt,worth＄216millionin2002～2003,ABARE2004)consistofblacklipabalone(HaliotisrubraLeach,1814).AssuchH.rubrais consideredasanimportantmarineresourcewithin Australia.Likemanyabalonespecieswor…     

Seamount gravity anomaly modelling with variably thick sediment cover

Inversion modelling of marine gravity anomalies to derive predicted seafloor topography has provided significant advance in delineating deep-ocean bathymetry where the seafloor both conforms to the half-space cooling model of seafloor spreading, and largely sediment-free. Similar modelling for elevated ridges and seamounts, that are formed by processes other than seafloor spreading and/or have proximal sediment sources (e.g., continental margins and volcanic arcs), have significantly higher errors when validated against modern shipborne echo-sounding data. A three-dimensional, five-layer gravity model is emulated for the cases of both synthetic and real seamounts, with varying degrees of sediment burial, to establish the sensitivity of variable sediment cover as a source of error. A simple `Gaussian' seamount with base radius of 30 km, 2000 m of relief, has a maximum 140–160 mGal anomaly, that decreases to 50 mGal with the addition of 1 km of sediment cover with simple `flood' geometry. Complete burial, with a typical sediment density of 2300 kg m^–3, results in a 120 mGal difference from a sediment-free seamount model. Increasing sediment density results in an exponential decay of the seamount anomaly. More complex synthetic geometries of varying basement relief and sediment thickness show that the anomaly amplitude remains significant, especially where the latter is >700–800 m thick. For the real case, seamounts of the Three Kings Ridge (northern New Zealand) imaged with seismic reflection data, with varying degrees of sediment cover of up to 1 km, when modelled both with and with-out the inclusion of a sediment layer, typically have rms differences of 30 mGal between observed and modelled gravity anomalies. Significantly, the rms errors are reduced by 50% with the inclusion of a sediment layer that corresponds to a reduction of predicted seafloor topography rms errors of 192–684 m to 78–360 m.     

The impact of tributyltin from large vessels on dogwhelk (Nucella lapillus) populations around Scottish oil ports

Surveys of imposex in dogwhelk (Nucella lapillus) populations around oil terminals in Sullom Voe (Shetland) and Scapa Flow (Orkney) have confirmed that the effects of tributyltin (TBT) released from large vessels in coastal waters can be recognised. The impact of large vessels can be separated from the effects of other inputs of TBT in the areas studied. The intensity of the effects was greater in Shetland than in Orkney, reflecting the higher exposure to TBT resulting from the greater volume of traffic using the Shetland terminal, and the more restricted water exchange in that area. A large proportion of the female dogwhelks in Sullom Voe were unable to reproduce. Females in Scapa Flow were still able to reproduce, except for a small proportion close to two fish farm sites where TBT net-treatments were used prior to 1987.     

Contributions to atmospheric methane by natural seepages on the U.K. continental shelf [Mar. Geol. 137 (1997) 165–189]

Natural gas seepages occur on the United Kingdom's continental shelf and although published reports suggest that they are very rare, the petroleum industry has identified, but not publicly reported, many more. There is also very little data on the flux of gas from seabed seepages, and even less on the contribution of seepages to atmospheric concentrations of gases such as methane.

Potential gas source rocks include Quaternary and Tertiary peats as well as petroliferous source rocks such as the Carboniferous Coal Measures and the Upper Jurassic Kimmeridge Clays. There are also other organic-rich sediments which are potential source rocks. Together these cover a considerable part of the U.K. continental shelf.

Analogue seismic reflection (pinger) profiles acquired during the British Geological Survey's regional mapping programme have been reviewed to identify water column targets including fish and plumes of gas bubbles. The ability to distinguish targets is critical to an assessment of the distribution of gas seepages. Both theoretical predictions of target identity and the habits of shoaling fish have been investigated in order to identify a method of distinction.

Data from seabed seepages and measurements of seepage rates have been used to establish likely ranges of gas flux rates and the sizes of gas bubbles. The likelihood that a rising bubble will survive and escape into the atmosphere is determined primarily by bubble size and water depth; methane, the principal constituent of seepage gas, is relatively unreactive and sparingly soluble.

The studies have enabled a new estimate of the distribution of gas seepages on the U.K. continental shelf, and of the contribution to atmospheric methane levels. The results suggest that natural gas seepages are significantly more important as a source of methane than had hitherto been established. It is estimated that between 120,000 and 3.5 mtonnes of methane per year come from a continental shelf area of about 600,000 km². This represents between 2% and 40% of the total United Kingdom methane emission. It is suggested that similar contributions arise from other continental shelf areas worldwide, and that geological sources of atmospheric methane are more significant than is generally acknowledged.